Method of Synthesizing Phosphide Materials

ABSTRACT

The subject of the invention is a method of synthesizing a compound M x P y  where M is an element belonging to one of columns II to XV of the Periodic Table of the Elements or to the family of lanthanides or to the family of actinides, characterized in that it includes the reaction of x moles of compound comprising the element M in its oxidation state 0 with y/4n moles of compound (P 4 ) n . 
     The method of the invention may be carried out at a temperature much lower than those necessary in the methods of the prior art. It also allows low-temperature formation of nanoparticles and stoichiometric reaction control. 
     The applications of this method are numerous: magnetic ferro-magnets magnets for MnP and FeP; hydrodesulfurization catalysts for Ni 2 P; luminescent nanoparticles for biological applications; microelectronics and optoelectronics for InP; and electronics for GaP. The latter two phosphides are also used in the photovoltaic energy field.

The invention relates to a novel method of synthesizing M_(x)P_(y) compounds starting from pure phosphorus. The compound M may be a metal or any other element belonging to one of columns II to XV of the Periodic Table of the Elements, or even any other element belonging to the family of lanthanides or actinides.

Such materials are of interest in very varied fields of applications, which are all those of known metal phosphides. Notably, the following may be mentioned: magnetic ferro-magnet applications for MnP and FeP; hydrodesulfurization catalyst applications for Ni₂P; luminescent materials compatible with biological media; microelectronics and optoelectronics for InP; and electronics for GaP. The latter two phosphides are also used in the photovoltaic energy field.

Various methods of synthesis have been proposed hitherto for synthesizing metal phosphides, including FeP, MnP, Ni₂P, PtP₂, InP and GaP, using synthesis intermediates and generally proposing a double thermolysis of a metal precursor and a phosphorus atom donor.

Thus, the synthesis of Ni₂P may be mentioned, which comprises the thermal decomposition of Ni(acac)₂ at 210° C. and then the reaction with a P(n-octyl)₃ phosphine compound at 330° C. This temperature of 330° C. is necessary for the decomposition of P(octyl)₃ which serves as a “phosphorus atom donor by breaking phosphorus-carbon bonds” as described in the publication “Generalized Synthesis of Metal Phosphide Nanorods via Thermal Decomposition of Continuously Delivered Metal-Phopshine Complexes Using a Syringe Pump” by Jongman Park, Bonil Koo, Ki Youl Yoon, Yosun Hwang, Misun Kang, Je-Geun and Taeghwan Hyeon, J. Amer. Chem. Soc. 127, 8433-8440 (2005). No control of the stoichiometry is therefore possible (P(n-octyl)₃ is used as solvent for the reaction). It is therefore possible to obtain only the M_(x)P_(y) phase stable at this temperature, namely Ni₂P in this case.

In the case of Pt (which is in the same column as Ni), this synthesis results in PtP₂, and therefore an inverse stoichiometry. Thus, it is not possible with this type of method to obtain a compound of different stoichiometry.

FeP and MnP syntheses follow a similar principle. The method is limited by the temperature of decomposition of P(n-octyl)₃, which is above 330° C.

As regards the synthesis of InP, it is known to carry out the thermal decomposition of various indium precursors of the CpIn (Cp=cyclopentadienyl), Cp*In (where Cp*=pentamethylcyclopentadienyl) or In(C(CH₃)₃)₃ type into In(0) nanoparticles followed by a reaction with P(SiMe₃)₃, which precursor is hydrolyzed in situ to PH₃, as described in the publication “Growth of InP Nanostructures via Reaction of Indium Droplets with Phosphide Ions: Synthesis of InP Quantum Rods and InP-TiO₂ Composites” by Jovan M. Nedeljkovic, Olga I. Micic, S. Philip Ahrenkiel, Alex Miedaner and Arthur J. Nozik, J. Amer. Chem. Soc. 126, 2632-2639 (2004). The compound P(SiMe₃)₃ is not only very expensive, as its synthesis is tricky and extremely dangerous, but it also hydrolyzes very easily in air to PH₃, which is an extremely toxic substance (fatal in a very low concentration in air: CL50 [ppm/1h]=20).

In this context, the present invention provides a method of synthesis employing a stoichiometric reaction giving an excellent reaction yield without loss of initial compounds, contrary to the methods of synthesis in the prior art. The method of the present invention thus makes it possible to obtain a group of M_(x)P_(y) stoichiometries, corresponding to the phases that are thermodynamically stable at the reaction temperature and in particular in the 10° C.-100° C. range.

More precisely, the subject of the present invention is a method of synthesizing a compound M_(x)P_(y) where M is an element belonging to one of columns II to XV of the Periodic Table of the Elements or to the family of lanthanides or to the family of actinides, characterized in that it includes the reaction of x moles of compound comprising the element M in its oxidation state 0 with y/4n moles of compound (P₄)_(n).

According to one embodiment of the invention, the reaction of x moles of compound comprising the element M in its oxidation state 0 with y/4n moles of compound (P₄)_(n) is carried out in the presence of a weakly coordinating solvent in order to allow M_(x)P_(y) nanoparticles to form.

According to one embodiment of the invention, the compound (P₄)_(n) is white phosphorus P₄.

Advantageously, the compound (P₄)_(n) is dissolved in toluene in order to carry out the synthesis according to the invention.

According to one embodiment of the invention, the element M is Ni, n being equal to 1, x being equal to 2 and y being equal to 1.

According to one embodiment of the invention, the Ni-containing compound is Ni(C₈H₁₂)₂ in which Ni is in its oxidation state 0.

According to one embodiment of the invention, the reaction is furthermore carried out in the presence of oleylamine allowing Ni₂P nanoparticles to be formed.

According to one embodiment of the invention, where the element M is In, the In-containing compound is (C₅H₅)In, but it may also be (C₅(CH₃)₅)In.

The invention will be better understood and other advantages will become apparent on reading the following description, which is given by way of non-limiting example.

In the method of synthesis according to the invention the advantage of using white phosphorus P₄, or even red phosphorus (P₄)_(n), lies also (in addition to the advantage of a stoichiometric reaction with a high yield) in the fact that it is possible to carry out what is called a “gentle” reaction, i.e. at low temperature and therefore with a large energy saving. This is because phosphorus P₄ possesses an arrangement of four low-energy bonds in a tetrahedron, which are therefore easily capable of being broken so as to create bonds with other compounds. The present invention therefore takes advantage of the weakness of P—P bonds in the reactant P₄, allowing reaction temperatures that are much lower than those observed in the syntheses already reported (around 300° C. to decompose P(n-octyl)₃ for example).

Likewise, according to the method of the invention, it becomes possible to form crystalline nanoparticles at low temperature.

The invention provides a very simple method of synthesizing metal phosphides, including Ni₂P, InP, GaP, FeP and MnP, using the base product of phosphorus chemistry, namely white phosphorus P₄. This method therefore differs substantially from the various methods for obtaining the above species. Furthermore, this method is general—it depends only on the judicious choice of the metal precursor. Thus, the low-temperature thermal decomposition of said precursor, in the presence of P₄ and solvent/stabilizing ligand, makes it possible to obtain metal phosphide nanoparticles.

The reaction of a metal precursor (source of M), dissolved in a suitable solvent, with a chosen P₄ stoichiometry, the P₄ itself being dissolved in toluene, at room temperature or any other temperature between 10° C. and 300° C., results in the almost quantitative formation and in the precipitation of an amorphous M_(x)P_(y) solid (the stoichiometry of which corresponds to the phase stable at this temperature).

EXAMPLE OF A METHOD ACCORDING TO THE INVENTION FOR SYNTHESIZING THE COMPOUND Ni₂P

The compound Ni(COD)₂ (0.5 mmol) was dissolved in an inert atmosphere in toluene and the solution was stirred. More precisely, the Ni(COD)₂ was bis(1,5-cyclooctadiene)Ni, the structure of which is as follows:

A solution of P₄ (0.06 mmol) dissolved in toluene was then added drop by drop, resulting in the instantaneous formation of an amorphous black solid comprising Ni₂P. This was isolated by simple filtration, washed and dried. X-ray analysis on the powder revealed the amorphous character of the material. Annealing the material led to the formation of crystalline Ni₂P, as attested by X-ray powder diffraction.

To obtain Ni₂P nanoparticles, a particular, weakly coordinating, cosolvent was used, this notably may be oleylamine, having the following chemical formula: CH₃(CH₂)₇CH═CH(CH₂)₈NH₂. Synthesis of the nanoparticles was carried out as follows:

The Ni(COD)₂ compound (0.5 mmol) was dissolved in a weakly coordinating solvent (oleylamine for example, at 5 mmol) and heated at 100° C. for two hours. The dissolved compound P₄ (0.06 mmol) was then added and the solution heated at 100° C. for one hour. The Ni₂P nanoparticles were then precipitated using a polar solvent, washed and redispersed in a nonpolar solvent. Typically, the polar solvent may be acetone and the nonpolar solvent may be hexane.

Thus, in the example presented relating to nickel, by judiciously choosing the precursor it is possible to synthesize nickel nanoparticles in the oxidation state 0: Ni(0) at low temperature (100° C.). For comparison, it has been reported that Ni(acac)₂ (where acac=acetylacetonate) decomposes at temperatures above 210° C. in the presence of a coordinating solvent, to form Ni(0) nanoparticles according to the prior art.

EXAMPLE OF A METHOD ACCORDING TO THE INVENTION FOR SYNTHESIZING THE COMPOUND InP

The compound (C₅H₅)In was dissolved in a solvent (toluene for example) at room temperature and the solution was stirred. The compound (C₅H₅)In may easily be obtained from InCl and LiC₅H₅ dissolved in diethyl ether.

The compound (C₅(CH₃)₅)In may just as well be used.

To obtain InP nanoparticles, a particular, weakly coordinating, cosolvent was used, which notably may be oleylamine of the following chemical formula: CH₃(CH₂)₇CH═CH(CH₂)₈NH₂. The synthesis of nanoparticles was carried out as follows:

The compound (C₅H₅)In (0.5 mmol) was dissolved in a weakly coordinating solvent (for example oleylamine, 5 mmol) and heated at 70° C. for one hour. The dissolved compound P₄ (0.5 mmol) was then added and the solution heated at 100° C. for one hour. The InP nanoparticles were then precipitated using a polar solvent, washed and redispersed in a nonpolar solvent. Typically, the polar solvent may be acetone and the nonpolar solvent may be hexane. 

1. A method of synthesizing a compound M_(x)P_(y) where M is an element belonging to one of columns II to XV of the Periodic Table of the Elements or to the family of lanthanides or to the family of actinides, characterized in that it includes the reaction of x moles of compound comprising the element M in its oxidation state 0 with y/4n moles of compound (P₄)_(n).
 2. The method of synthesis as claimed in claim 1, characterized in that the reaction of x moles of compound comprising the element M in its oxidation state 0 with y/4n moles of compound (P₄)_(n) is carried out in the presence of a weakly coordinating solvent in order to allow M_(x)P_(y) nanoparticles to form.
 3. The synthesis method as claimed in either of claims 1 and 2, characterized in that the compound (P₄)_(n) is white phosphorus P₄.
 4. The method of synthesis as claimed in claim 3, characterized in that the element M is Ni, x being equal to 2 and y being equal to
 1. 5. The method of synthesis as claimed in claim 4, characterized in that the compound comprising Ni is Ni(C₈H₁₂)₂ in which Ni is in its oxidation state
 0. 6. The method of synthesis as claimed in one of claims 1 to 5, characterized in that the reaction is carried out in the presence of toluene.
 7. The method of synthesis as claimed in either of claims 5 and 6, characterized in that the reaction is furthermore carried out in the presence of oleylamine allowing Ni₂P nanoparticles to be formed.
 8. The method of synthesis as claimed in one of claims 1 to 3, characterized in that, when M is In, x is equal to 1 and y is equal to 1, the In-containing compound is (C₅H₅)In.
 9. The method of synthesis as claimed in one of claims 1 to 3, characterized in that, when the element M is In, the In-containing compound is (C₅(CH₃)₅)In. 